The present disclosure relates generally to power supplies for light emitting diodes (LEDs), especially for power supplies with the ability of suppressing or reducing audio noise.
This is an era that power consumption and efficiency are important issues for almost every device in this modern world. LEDs, because of their excellent power efficiency and compact device size, have become more and more popular in lighting markets. For example, the cold-cathode fluorescent lamps (CCFL) in the back-light modules of liquid-crystal-display (LCD) panels have largely been replaced by LEDs.
FIG. 1 illustrates back light module 8 with LEDs and a power supply. The power supply of FIG. 1 has two stages: voltage-controlled stage 4 and current-controlled stage 6. As shown in FIG. 1, voltage-controlled stage 4 is a booster, in which power controller 18 alternatively turns on and off power switch 15 to store electric power in inductive device PRM and to release the stored electric power such that output voltage VOUT with required specifications is built up at output node OUT connected to LEDs. Current controller 20 in current-controlled stage 6 majorly balances the currents through the LED chains, such that the currents are substantially the same in amplitude and all LED chains illuminate evenly.
To adjust the brightness of an LCD panel, back light module 8 could receive a dimming signal VDIM to substantially control the lighting of the LED chains. Generally speaking, when dimming signal VDIM is asserted, the LED chains illuminate, and when dimming signal VDIM is deasserted, the LED chains stop illuminating. The duty cycle of dimming signal VDIM, that is, the asserted time in proportion to the cycle time, determines the intensity of lighting felt by human eyes.
FIG. 2 shows dimming signal VDIM at dimming node DIM, gate signal VGATE at gate node GATE, current IIN flowing into inductive device PRM from input node VIN, and output voltage VOUT at output node OUT. During the dimming-ON period when dimming signal VDIM is asserted, power controller 18 outputs gate signal VGATE to alternatively turn on and off power switch 15. Meanwhile, current IIN is drained from input node VIN to build up output voltage VOUT. Current controller 20 also conducts and spreads current IIN through LED chains to illuminate.
During the dimming-OFF period when dimming signal VDIM is deasserted, power controller 18 deasserts gate signal VGATE, current IIN is about 0 A, and output voltage VOUT might slightly ramp down over time due to some leakage current. Current controller 20 could cut the current paths through the LED chains so that the LED chains stop illuminating.
From the perspective of voltage-controlled stage 4, it can be found from the signals in FIG. 2 that switching between the dimming-OFF period and the dimming-ON period is equivalent, per se, to switching between no load and heavy load. Even if the frequency of dimming signal VDIM might be as low as 200 Hz within the frequency range hardly heard by human, the load transition is so large that current IIN could has considerable energy allocated in some frequencies harmonic to the frequency of dimming signal VDIM and cause inductive device PRM to generate noise, which is unpleasant to human and should be erased or diminished in consumer products.